1. Field of the Invention
This invention relates to a CVD (Chemical Vapour Deposition) method, and more particularly to a CVD method in which a second metallic film is formed on a first metallic film in a via hole or contact hole of an insulating film formed on a substrate of a wafer.
2. Description of the Prior Art
An apparatus which is used for a CVD method of prior art is shown in FIG. 1. Referring to FIG. 1, an inlet 52 and an outlet 53 are made respectively in the side walls of a horizontal reaction chamber 51 which is evacuated. A wafer holding plate 54 is horizontally arranged in the reaction chamber 51. Plural wafers 55 are vertically held at constant pitches by the wafer holding plate 54. An electrical heater 56 is arranged around the reaction chamber 51.
WF.sub.6 as gas containing metallic element and H.sub.2 as reduction gas are introduced into the reaction chamber 51 from the inlet 52, and they are exhausted outwards from the outlet 53. The electrical heater 56 heats the reaction chamber 51. The temperature of the inner wall of the reaction chamber 51 rises. The heat from the inner wall is transmitted through the reaction gases WF.sub.6 and H.sub.2 to the wafers 55. The wafers 55 are thus heated, and the reaction gases WF.sub.6 and H.sub.2 chemically react on the wafers 55. Metallic films are formed on the wafers 55.
FIGS. 2A and 2B show the details of the wafer 55. The wafer 55 has the construction of FIG. 2A or FIG. 2B. Referring to FIGS. 2A and 2B, an insulating film 60 of SiO.sub.2 is formed on a substrate 55a of the wafer 55. Via holes or contact holes 60a are formed in the insulating film 60. A first metallic film 58 or 58' of W or Al or Aluminum alloy is formed in said via hole or contact hole 60a. It is inferred that the chemical reactions represented by the following formulas (1) and (2) are effected on the metallic film (surface) 58 or 58': EQU 3H.sub.2 .fwdarw.6H (1) EQU WF.sub.6 +6H.fwdarw.6HF+W (2)
W films grow with time. Second metallic films 59 are formed on the first metallic films 58 in the via hole or contact hole 60a as shown in FIG. 3A and 3B.
Generally, the growing rate G of the W film is expressed by the following equation: EQU G=A[H.sub.2 ].sup.1/2 exp (-Ea/kTm) (3)
where A represents a positive constant, [H.sub.2 ] a hydrogen concentration, Ea activation energy, k Boltzmann's constant, and Tm the surface temperature of the growing metallic film.
In the above described CVD method of prior art, the surface temperature Tm of the first metallic film 58 or 58' in the via hole 60a and the surface temperature Ti of the insulating film 60 ar nearly equal to each other. As understood from the equation (3), the growing rate G of the W film increases with the surface temperature Tm. However, the surface temperature Ti increases with the surface temperature Tm, in the prior art CVD method. Accordingly, at high deposition temperature, the metallic films 58, 58' and 59 of W are formed not only in the via hole or contact hole 60a, but a metallic nucleation 61 are formed also on the insulating film 60, as shown in FIGS. 3A and 3B, since the above chemical reactions represented by the formulas (1) and (2) are effected both in the via hole or contact hole 60a and on the insulating film 60. That is disadvantageous.
FIG. 4 is an enlarged view of a part of the wafer 55 shown in FIG. 3B. In the growing process of the metallic film 59, metal W encroaches the interface between the Si substrate 55a and the insulating film 60. Thus, the encroachment 70 is formed. In some cases, voids 71 are formed in the substrate 55a.
These inventors inferred that the turbulent flow or natural convection in the vicinity of the wafer 55 promotes the formations of the encroachment 70 and voids 71, in the prior art CVD method. However, control parameters are only two internal parameters, which are pressure and flow rate of reaction gas. Accordingly, it is not possible to control the turbulent flow and natural convection from the outside, and the encroachment and voids cannot be suppressed. The metallic film formation of the prior art has some problems on the reproducibility, controllability and uniformity in the wide range of the pressure and flow rate.
Further in the prior art method, the reaction components are diffused into the whole space of the reaction chamber. They adhere to the inner walls and the inspection window. Such problems occur from that fact that dust particles are made, and they are mixed as impurities into the film.
Film formation at low temperature and low concentration is considered for suppressing the encroachment and voids. However, the growing rate becomes low, such as scores of angstroms .ANG./min. (Broadkent Metal. J. Electrochem. Soc. 131, (42)(1984); Blower. VMIC (1985)). For example, about two hours are required for filling a via hole or contact hole of 1 .mu.m depth.